Arrhythmias are abnormal heart rhythms that may cause the heart to function less effectively. Atrial fibrillation (AF) is the most common abnormal hear rhythm. Research has indicated that as many as ninety-percent of all thrombi formed during AF originate in the left atrial appendage (LAA). The LAA is a remnant of an original embryonic left atrium that develops during the third week of gestation. It is located high on the free wall of left atrium. Long, tubular, and hook-like in structure, the LAA is connected to the left atrium by a narrow junction, referred to as the ostium. The precise physiological function of the LAA remains uncertain.
The high rate of thrombus formation in the LAA is believed to be attributable to its physical characteristics. Blood easily stagnates and clots in the long, tubular body of the LAA or at its narrow ostium. In contrast, the right atrial appendage, which is a wide, triangular appendage connected to the right atrium by a broad ostium is infrequently the site of thrombus formation. Thrombus formation in the LAA is further promoted by the numerous tissue folds (i.e., crenellations) on its interior surface. These crenellations are particularly hospitable to blood stagnation and clotting, especially when the heart is not functioning at maximum capacity. Thrombi formed in the LAA can re-enter the circulation upon conversion of AF to normal rhythm (i.e., cardioversion).
Obliteration and occlusion are controversial because of the uncertain physiological role of the LAA. Reports have suggested that obliteration of the LAA may decrease atrial compliance and diminish peptide hormone atrial natriuretic factor secretion. Furthermore, while properly positioned filter devices prevent migration of thrombi into the circulatory system, such devices cannot inhibit thrombus formation within the LAA. Consequently, in the event a filter device is dislodged or ineffectively sealed against the LAA ostium, clots held at the LAA ostium by the filter can be released into the circulation.